This invention relates to cylinder deactivation engines having switching valve actuators for deactivating selected cylinders of an engine. In particular, the invention involves modification of camshaft cam timing for improved operation of a cylinder deactivation engine.
It is known in the art relating to cylinder deactivation engines to provide lost motion hydraulic valve lifter or lash adjuster devices with sliding pin latching mechanisms, hereafter called latching pins. These lost motion devices may be referred to as switching valve actuators and can include both collapsible valve lifters with internal lash adjusters and stationary collapsible lash adjusters used as pivot members in engine valve gear.
The latching pins of such switching valve actuators require a mechanical lash component to insure free movement of the latching pins. As a result, the mechanisms are designed so that the hydraulic lash adjuster or lifter cannot compensate for the required mechanical lash. To prevent valve train noise, this mechanical lash must be taken up by additional camshaft cam ramp height added to the opening and closing side hydraulic ramps in each valve line containing a lost motion (switching) valve actuator. Further, to prevent noise with all the expected variations in the actual mechanical lash in a population of production parts, the additional ramp must be equal to the maximum allowed mechanical lash plus any wear expected during the life of the engine.
While the increased ramps will remove the possibility of valve train noise due to the mechanical lash component of the switching valve actuators, they will also increase the valve overlap area in every valve line having a switching valve actuator, except in the case of a maximum mechanical lash condition, due to the additional ramp height remaining after a less than worse case lash is removed. The additional overlap area can lead to a significant degradation in combustion quality during lightly loaded conditions and at idle and result in a perceived roughness by an operator. A method of overcoming these potential negative effects with a minimum of engine modification is accordingly desired.
The present invention provides a modification of engine cam timing which substantially reduces or eliminates increased valve overlap area resulting from the mechanical ramps previously discussed. In a typical engine, the timing of exhaust valves relative to intake valves within each cylinder is designed to be identical across all cylinders of the engine. If the valve timing is altered from cylinder to cylinder, it is possible to vary the resulting overlap in each cylinder as well.
The present invention provides an advance of exhaust cam timing for the cylinders with switching valve actuators while keeping the intake valve timing fixed. The advanced exhaust cam timing reduces the additional overlap created by the mechanical lash. The remaining cylinders without switching valve actuators are not changed. Advance of the exhaust valve timing is chosen because engine performance does not change significantly as exhaust valve timing is changed within a certain limited range of values, while even small changes in intake cam timing can significantly affect engine torque output.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.